V22.0436 - Prof. Grishman

Combining data paths

We have seen the data paths needed for the individual types of
instructions.
To construct a complete data path for our machine, we need to combine
these
data paths, using multiplexers where necessary (Fig. 5.11).

Data paths and control

These data paths require various control signals: register numbers,
read/write
control for memories and registers, function for ALU, select lines for
registers. The values of these control signals must be determined based
on the instruction being executed.

The instructions we are considering (lw, sw, beq, add, sub, and, or,
slt) are stored in three instruction formats (Fig. 5.14). The opcode of
the instruction is stored in the high 6 bits (bits 26 to 31). However,
the R-type instructions all are assigned opcode 0, and are
differentiated
by the "function" field, bits 0 to 5 of the instruction.

The register numbers are stored in the same place in all these
instructions,
and so we can connect the instruction fields (the output of the
instruction
memory) directly to the register number inputs on the register file. We
need one multiplexer, however, since the number of the register to
write
sometimes appears in the rd field, bits 11 to 15 (for R-type
instructions)
and sometimes in the rt field, bits 16 to 20 (for load instructions).
This
produces the circuit shown in Fig. 5.15.

Control Logic

We must construct logic to control the ALU functions plus the following
control lines (see Fig. 5.16): MemRead, MemWrite, ALUSrc, RegDst,
RegWrite,
PCSrc, and MemtoReg.

We construct a truth table in which the inputs are the instruction
---
specifically, the opcode and function fields --- and the outputs are
the
control signals and ALU function. P&H do this in two steps: The
control
signals (exclusive of ALU function) are determined by the opcode alone.
The dependence is shown in Figure 5.18. P&H then define a two-bit
signal
called ALUOp, which is a function of the opcode. The ALU control signal
truth table is then based on two inputs: ALUOp and the function field
(Figure
5.13).